Frequently ultraviolet rays are used for producing photochemical reactions or for curing paints, inks and the like. Ultraviolet rays in a wavelength range of approximately 280 to 400 nm are effective in curing paints, inks and the like.
The radiation source for the ultraviolet rays with such a wavelength range is conventionally constituted by a high pressure mercury vapor lamp. The radiation light of a high pressure mercury vapor lamp consists of a plurality of line spectra, which are in a rather wide wavelength range. However, it is not effective to use a high pressure mercury vapor lamp for photochemical reactions, for curing paints and the like, because the effective wavelength range for such purposes is 280 to 400 nm.
It is therefore becoming ever more frequent practice to use a metallic vapor discharge lamp, in which in an arc tube of a high pressure mercury vapor lamp together with the mercury use is made of another metal, i.e. a metal iodide, metal bromide, metal chloride or metal halide comprising a composition of said metals, encapsulated as light-emitting materials and in this way the radiant quantity of the effective wavelength range is increased. In particular, a metallic vapor discharge lamp in which iron is encapsulated together with the mercury is favorable for photochemical reactions or curing paints and the like due to its continuous radiation spectrum in a wavelength range of 350 to 400 nm. However, if a metallic vapor discharge lamp in which iron is encapsulated remains in operation for a long time, a thin film is formed due to the adhesion of the iron to the inner wall of the arc tube.
More particularly in order to increase the productivity in a drying process for paints, inks or the like, it is desirable to have a metallic vapor discharge lamp, which emits stronger, effective ultraviolet rays.
However, if in order to meet this need, a larger amount of iron is encapsulated, within a relatively short time a thin iron film is formed in an even greater quantity on the inner wall of the arc tube. It is therefore considered disadvantageous that the thin iron film formed on the inner wall of the arc tube prevents the permeability of the effective ultraviolet rays through the arc tube wall and consequently the intensity of the ultraviolet rays in the wavelength range of 280 to 400 nm is reduced because of the formation of a thin iron film after a lighting period of a few dozen hours. To eliminate the aforementioned deficiency, a metallic vapor discharge lamp is proposed in which, together with the mercury, iron and also a further metal are encapsulated, so that no thin iron film is formed. As is known, the formation of a thin iron film on the inner wall of an arc tube can be prevented if an addition is made of e.g. lead (Japanese Utility Model SHO 54-15503), tin (Japanese Patent SHO 58-18743), magnesium (Japanese Published Patent Application SHO 62-80959), cadmium (Japanese Published Patent Application HEI 1-161655), manganese (Japanese Published Patent Application HEI 1-128345) or the like.
However, of late there has been a considerable need for a lamp, which has a strong radiation in the wavelength range 280 to 400 nm, whilst at the same time having a longer life. However, if the aforementioned metals are added to a lamp, it has been found that a thin iron film is still formed on the inner wall of the arc tube if the lighting period is extended. This means that the addition of the aforementioned metals to a lamp, in which mercury and iron are encapsulated, cannot completely effectively prevent the formation of a thin iron film and instead only reduces the speed with which such a film is formed.
The above-described lamps are conventionally used in such a way that the outside of the arc tube is cooled to approximately 850.degree. C. with cooling air within a lamp housing. It has been found that the formation of a thin iron film can be significantly reduced if the maximum temperature of the outside of the arc tube of the metallic vapor discharge lamp is kept at approximately 800.degree. C., by increasing such a cooling action. By reducing the maximum temperature of the outside of the arc tube to approximately 800.degree. C., there is also a reduction in the temperature of the coolest portion of the inside of the arc tube, which reduces the intensity of the emission through the luminescing of the iron and consequently also reduces the radiation intensity of the ultraviolet rays used for curing paints and the like even though there is no formation of a thin iron film.